Dear Dev Team,
Clearly you have made up your minds on the one character per island, so pick a day to implement the changes and be done with it; courtesy would dictate at least a few weeks to prepare for the actual change. Of course, this change will grind to a halt, for a time, all of the recent diplomatic and military activity on some of the "stale" islands (like Atamara), but hey, why let that get in the way?
Why you are at it, you should make a change that would actually have a larger effect on stagnation, and put in a natural death code to prevent all of those 100+ year old characters who occupy realm council and Duke positions. There is nothing meaningful left to accomplish RP-wise with characters of that age, and it would actually give new players a chance to rise to positions or power, especially the coveted Duke/Duchess position, where they can split to form new realms. And yes, I know of at least one Dev who had a 99 year old Duchess, but I am sure they were the exception to the "nothing meaningful left to RP" crowd. In my experience since joining the game almost 10 years ago, these old characters are just as likely, if not more so, than a second character on an island, to be a "drone." Combine this with the one player per island, and a player's own "replacement" can't be groomed from their character list.
While I am not a fan of the character limit change, mostly given the timing related to the recent events on Atamara, if it is inevitable, then announce the change and be consistent in taking steps to reduce stagnation and promote the enjoyment of new players by having a mass plague on all the islands that kills characters who are, at a minimum, over 100, with 85 probably being a better cut off.